Hidden Singles
The digit hiding in plain sight.
Hidden Singles is the complement to Obvious Singles: instead of finding a cell narrowed down to a single candidate, you find a digit narrowed down to a single cell. It occurs when a digit appears as a candidates in only one cell within a row, column, or 3×3 box.
Because Sudoku requires every digit to appear exactly once in each region, and this digit has only one legal home left in the region you're scanning, it must go there regardless of what else shares the cell. Spotting a hidden single means shifting your attention from individual cells to individual digits: instead of asking what a cell could hold, you ask where a digit could still go across an entire region. That shift in perspective is what makes hidden singles easy to overlook and essential to check for once obvious singles run out.
How It Works
The "hidden" refers to the fact that the candidate is not alone in its cell. Other candidates share the cell, masking the fact that this particular digit has nowhere else in the region to go.
- Choose a digit.
- Scan a row, column, or box.
- If the digit appears as a candidate in only one cell in that region — even if that cell has other candidates too — place it there.
- Eliminate all other candidates from that cell.
Hidden singles require you to think about digits, not cells. Instead of asking "what goes in this cell?" you ask "where in this region can this digit go?" It is a different mode of thought — and a necessary one.
Example
Below board presents an example of a hidden single on digit 7, tucked inside a box among cells that each carry other candidates.
Every empty cell in this box has several candidates, and 7 is never the only one in its cell. But look across the whole box for 7 specifically: it appears in exactly one cell's notes. That cell must be 7 — not because it lacks other candidates, but because nowhere else in the box can 7 go.
On Evil Boards
Spotting hidden singles is a very useful skill in Sudoku. It reveals how the puzzle actually works — not as a collection of cells to fill, but as a system of digits seeking their only possible homes.
Hidden singles are rare in the early game on an Evil board. You will apply more advanced techniques — like hidden pairs or pointing pairs — to manufacture the conditions in which they appear.
Some sources use "hidden single" and "last remaining cell" interchangeably — same deduction, found here by reading candidate notes instead of scanning placed digits directly.